In recent years, cone-beam CT (CBCT) has been integrated with linear accelerators for volumetric imaging of the patient anatomy at radiotherapy treatments. Proposed or implemented usage of CBCT in radiotherapy includes patient setup based on bony structures and soft tissue, recalculation of patient dose, and adaptive radiotherapy. Under the assumption of periodic motion, CBCT projections can be sorted into sub-period phases that can be reconstructed separately to form static volumetric images for each respiratory phase. Although such four-dimensional CBCT scans provide the mean patient anatomy in each phase, variations in the respiratory cycles and non-periodic motion such as prostate motion are not resolved. The main focus in these CBCT applications has been on the reconstructed volumetric images, whereas the CBCT projections themselves are merely regarded as an intermediate step toward the reconstruction. In many institutions, the CBCT projections are deleted after creation of the volumetric images. However, when radiopaque markers are used as tumor surrogates, their positions on each CBCT projection contain valuable information about the tumor motion during the CBCT acquisition. The projections provide two-dimensional (2D) information of the tumor trajectory in a rotating coordinate system. Reconstruction of the actual three-dimensional (3D) tumor trajectory from the projections cannot be made without ambiguity because an infinite number of 3D trajectories result in the same sequence of projections.
The development of volumetric-modulated arc therapy has enabled efficient delivery of complex radiotherapy dose distributions within a single gantry rotation. The high conformity of the dose distribution to the target volume increases the accuracy requirements in the treatment delivery. In the case of prostate cancer this is especially true for hypofractionated radiotherapy, where recent studies have suggested that safe delivery may require real-time prostate position monitoring during treatment and correction strategies to compensate for occasional large intrafraction prostate motion.
The current state of the art has, in a clinical setting, combined real-time prostate position monitoring with motion compensation by periodic realignment of the treatment beam. The required target position signal during treatment delivery is provided by stereoscopic x-ray imaging, i.e. pairs of simultaneous x-ray images of implanted markers acquired every 30-60 sec.
Although still lacking integration with real-time motion compensation, intrafraction prostate position monitoring with stereoscopic x-ray imaging has also been implemented clinically for conventional gantry-mounted linear accelerators. The imaging has been performed with kV imager pairs that either rotate with the gantry or are mounted in stationary positions in the treatment room. Alternatively, intrafraction prostate position monitoring can be performed with implanted electromagnetic transponders.
While kV imager pairs are not standard equipment, many modern linear accelerators are equipped with a single gantry-mounted kV x-ray imager that enables x-ray imaging perpendicular to the MV treatment beam. Target position monitoring can be performed with stereoscopic imaging based on the kV imager and MV portal imaging. Use of portal images is attractive because it does not pose additional dose to the patient, but a high degree of beam modulation by the MLC leaves will often hinder target (surrogate) visibility on the portal images. Also, the implanted markers may be too small to be visible in MV images.
Accordingly, there is a need to develop a method to estimate the three-dimensional (3D) target position from the two-dimensional (2D) projections on a single imager and the 3D probability density function (PDF) for the target and apply the method for estimation of the 3D target trajectory from the projected 2D trajectory in a series x-ray images in very accurate trajectory estimations with estimation errors less than 1 mm for both prostate and tumors with respiratory motion. Further, there is a need to extend retrospective trajectory estimation to prospective real-time trajectory estimation in order to allow actions to be taken to account for the target motion (e.g. tracking or treatment interruptions).